1. Field of the Invention
This invention relates to mosaic needle print heads and more particularly to needle guide means for such print heads.
2. Prior Art
This invention is directed to that class of device which is utilized for guiding the printer needles of a mosaic needle print head such as is used in office, data or telex machines. The guide device is utilized to guide the elongated needles between the needle movement power means, generally plunger type magnets, and the printing zone of the print head adjacent where printing occurs. In the printing zone the print needles are brought close together in one or more aligned rows.
Depending upon whether the print head is constructed to reproduce upper case letters alone or lower case letters of shorter length, and whether the mosaic for forming an individual character is to be coarse or fine, a greater or lesser number of needles are required. In each instance a plurality of print needles which work in parallel to one another are required to form the vertical extent of the character to be printed. The needles form the horizontal extent of the character by being activated intermittently as the printing head moves along the print line. A standard format is a 7.times.5 arrangement requiring at least seven vertically disposed print needles with up to five activation points per character width for the formation of upper case letters.
In order to operate the print needles and cause them to move back and forth into and out of contact with the paper or other recording carrier being utilized, or into or out of contact with intermediate positioned ink means, power means are utilized. The normal power means are plunger type magnets which take up far more space than the height of the individual letters. Thus, the elongated printer needles which extend from the power means to the print zone must be guided from the print zone along divergent paths to the individual power means.
This necessity of guiding along divergent lines is particularly found when plunger type magnet systems are used as the power means. Such plunger type magnet systems have numerous inherent advantages which make them desirable for use in such mosaic needle printers, however, their large diameter provides a problematical power means arrangement problem since the individual needles exit from the magnet assembly, generally, in an axial center thereof.
Additionally, since the needles themselves are very thin, proper guiding of the needles from the power means to the print zone produces a number of problems. However, guiding is critical since a good guide in the zone intermediate the print zone and the power means has a decisive effect on the satisfactory operation of the overall printing head and upon the service life thereof. Thus, in recent years, considerable attention has been paid to this guiding problem and a number of suggested solutions have been attempted.
Among other things, it has been known to provide a guide device for such print needles in mosaic needle printers wherein all the drive means are aligned with their drive axes lined up on the print point. In such constructions the print needles then extend between the drive means and the print point in straight lines. It has been suggested to guide the needles at spaced intervals by support arms. However, such arrangements provide a problem in connection with the distance between the print head and the recording carrier. When the distance increases the image produced must, of necessity, become smaller since the needles are converging.
In order to eliminate this problem, it has become usual to guide the print needles in such a manner that they are parallel to one another adjacent the print zone and fan out from one another from the print zone back towards the spaced apart drive means. This requires guidance of the print needles along curved paths. However such curved paths provide further guidance problems since the print needles are now subjected to high bending stresses. It has also been suggested to provide spaced apart support arms along the length of the needles to provide a guide for the needles. In at least one known arrangement (British Pat. No. 1,418,219), the construction of such an assembly involves great care and precision in order to insure reasonably affective guidance characteristics for a relatively prolonged service life.
In contrast thereto, other devices are known wherein the print needles are guided over a major portion of the zone between the printing zone and the drive means. In one such arrangement (German laid open application No. 2,153,005), such guidance is obtained by cast guides. In order to provide the cast guide openings, master needles are provided during the casting process with a plastic block being cast around them. The production of such cast blocks is very problematical and creates a number of difficulties such as, for example, correct removal of the long thin master needles.
In at least one other suggested arrangmeent, the individual print needles are guided in guide tubes which are again provided by casting or molding.
It would therefore be an advance in the art to provide a needle guide assembly which is economical to produce, easy to assemble and disassemble, capable of providing adequate support to the needle wires and conductive of long print head life.